1. Field of the Invention
The present invention relates to a case having a cover, and more particularly to a mounting structure of the cover employed in the case.
2. Description of the Prior Art
A modern telephone has been developed to be multi-functional. As a result, in a panel of a main body of the case in such modern telephone, in addition to a set of dial keys and function keys, there are provided; a set of quick-dial keys; a memory key; and further there is provided a set of name plates in which a set of names of subscribers to be called, for example; names registered in the quick-dial keys, are to be entered. Further, on an upper portion of the main body in which the name plates are provided, there is provided a transparent cover. Since the names of the subscribers to be called are different in each of the users of the telephone set, it is necessary for the users to write such names on the name plates by themselves. Consequently, such cover should be mounted on the main body in an easily detachable manner.
In a main body of a conventional case, in order to mount such transparent cover in a detachable manner, there is provided a concave portion which receives the cover, each of the opposite side walls thereof being provided with a pair of holes, while each of the opposite side edges of the cover is provided with a pair of projections which corresponds to the pair of the holes of the concave partion of the main body. Mounting of the cover on the main body is carried out as follows: namely, the projections of one of the opposite side edges of the cover are inserted into the corresponding holes of the side walls of the concave portion of the main body, and then with the use of resiliency which the cover has the projections of the other side edge of the cover are inserted into the holes of the other side wall of the concave portion.
The back side of the cover thus mounted is supported by abutting surfaces of the bottom of the concave portion of the main body, while the cover is prevented from separated from the bottom of the concave portion by its projections having been inserted into the holes of the main body. The cover is also prevented from moving in directions parallel to the bottom surface of the concave portion because its edges are abutted against the walls of the concave portion of the main body.
However, such a conventional cover mounting structure has a following problem:
As described above, the cover is mounted on the main body by inserting the projections of one of its opposite side edges into the holes of one of the side walls of the concave pertion and then inserting its remaining projections of the other side edge into the remaining holes of the main body, thus the cover is firmly mounted on the main body by the resiliency of the cover. In order for the cover to be able to be firmly mounted on the concave portion of the main body, distance between the opposite side walls of the concave portion of the main body (referred as L) must be larger than the width of the cover (referred as l), but smaller than a sum of the above width l of the cover and a projecting height of the shortest one of the projections provided on the side edges of the cover (referred as m).
Further, since the resiliency of the cover is utilized in mounting the cover on the main body, it is necessary to set an inserted length (l+m-L) (that is, the width of the cover plus the height of the shortest projection minus the distance between the opposite walls of the concave portion) of the projection within a value determined by considering the restriction of the resiliency of the cover. Thus, in practice it is preferable to minimize this inserted length. However, if the inserted length is too short, it is impossible to firmly mount the cover on the main body, or wear of the projections caused by repetition of mounting/detaching operations will eventually make it impossible to firmly mount the cover on the main body. On the contrary, when the inserted length is too large, the cover and/or the main body are deformed beyond their elastic limit to cause permanent deformation of even breakdown. Particularly, in case that a transparent resin is employed as the material for the cover, and different material is employed for the main body, and these materials are largely different from each other in their linear thermal expansion coefficients, the inserted length is varied by an ambient operating temperature.
Suppose now that the ambient operating temperature varies in a range of from -20.degree. to +60.degree. C.; the main body is made of polystyrene resin (linear expansion coefficient .alpha.=10.2.times.10.sup.-5 /.degree.C.); the cover is made of polycarbonate resin (linear expansion coefficient .beta.=5.4.times.10.sup.-5 /.degree.C.); and widths of cover and concave portion are 100 mm (i.e., L=l=100 mm), then the variation in the inserted length is 0.4 mm at maximum.